The proposed research is directed towards understanding the molecular mechanisms regulating gene expression in a simple eukaryotic organism, the yeast Saccharomyces cerevisae. These studies involve genetic and biochemical analysis of various aspects of the yeast mating process. The mating process is controlled by the mating type locus, which can exist in oee of two states, a or alpha. These alleles control the production of and response to mating-type specific pheromones, alpha-factor, and a variety of other processes. From analysis of mutations in the mating type loci, we have proposed a hypothesis for determination of yeast cell type, in which the alpha mating type locus codes for at least two regulatory gene functions which control expression of genes unlinked to the mating type locus. One function (alpha1) codes for a positive regulator of alpha-specific functions; the other function (alpha10) codes for a negative regulator of alpha-specific functions. We shall continue tests of this hypothesis by isolation and characterization of additional mutations affecting the mating type loci and by isolation and characterization of new types of mutations predicted by this hypothesis. Although mating types a and alpha are stable to growth, rare interconversions from a to alpha and from alpha to a can be observed spontaneously. In addition, a gene (HO) exists which promotes this interconversion at high frequency. We have proposed a model (the "cassette model") in which all yeast cells contain unexpressed copies of the a and alpha mating type loci which can be activated by inserting copies of these blocs into the mating type locus through action of the HO gene. We shall continue our tests of this hypothesis and, in addition, determine why the silent copies are not expressed and provide informaiion on the mechanism of genetic transposition. This study of control of the mating process in yeast will provide information concerning the molecular bases by which genes are controlled and by which a stable switch is established and maintained in a eukaryotic organism.